1. Technical Field
This disclosure relates to a method of purifying a bioabsorbable polymer and, more particularly, to a method of extracting organic and inorganic impurities such as solvents, additives, monomers, oligomers, and the like, from a bioabsorbable polymer using supercritical fluid extraction (SFE).
2. Background of Related Art
A supercritical fluid is a dense gas that is maintained above its critical temperature, i.e., the temperature above which it cannot be liquified by pressure. Supercritical fluids exhibit a unique combination of solvent and transport properties and have been employed in the extraction of impurities from a wide variety of polymeric materials. See, e.g., U.S. Pat. Nos. 4,703,105, 4,892,931, 4,902,780, 4,918,160, 4,990,595, 5,049,647, 5,221,731, 5,229,486, 5,237,048, 5,248,763 and 5,250,658.
Bioabsorbable polymers have been employed in the fabrication of a broad range of implantable medical/surgical devices, prostheses, implants, and the like. After being implanted inside the body, a medical/surgical device fabricated from a bioabsorbable polymeric material will hydrolyrically degrade into harmless substances. Such polymers are extremely useful because they obviate the necessity for a subsequent surgical operation to remove the device after the healing process has been completed. Furthermore, such polymers exhibit predictable physical properties such as strength, toughness, etc. which are critical to their use in medical applications. However, because of their sensitivity to a number of environmental conditions, methods for purifying the polymers have generally been limited to those which do not expose the polymers to severe conditions, such as high temperatures and pressures.
One method for purifying bioabsorbable polymers includes contacting particles of impurity-laden bioabsorbable polymer, e.g., polyglycolic acid, with a flowing stream of a substantially dry, oxygen-free inert gas as disclosed in U.S. Pat. No. 3,565,869. The gas employed in this method, however, is not maintained under supercritical conditions of temperature and pressure. This method has been criticized (see U.S. Pat. No. 5,041,529) as a method which ineffectively removes impurities, requires several tens of hours to reduce residual monomer levels to below 2% and, furthermore, one which results in decomposition of the polymer.
Another method for purifying bioabsorbable polyesters includes dissolving the polymer in a solvent and thereafter contacting the solution with a precipitation agent under the effect of high shear forces in a turbulent shear field as disclosed in U.S. Pat. No. 4,810,775. A drawback to this method is that extremely flammable solvents and/or precipitation agents such as petroleum ether are frequently used.
Yet another method for purifying bioabsorbable polyesters includes treating the polyester under reduced pressure in a reaction system while maintaining the polyester in the molten state during the second half of the polymerization reaction as disclosed in U.S. Pat. No. 5,041,529.
The aforementioned methods do not employ a supercritical fluid in the purification of bioabsorbable polyesters.